STUDY OF ALKALINE BRECCIAS, DYKES AND SILLS FROM EASTERN CORDILLERA, BOLIVIA
The igneous rocks intruded to the meta-sedimentary rocks of Ordovician that consists of quartzites and slates. The breccias crop out in two hills and other bodies aligned from N to S. The subvolcanic bodies intruided during three pulses. The first of them intruded the breccia, while a posterior pulse formed the dikes and sills and finally, another intrusion of dykes/sills cuts to the dykes and breccias coming from the previous pulses.
Petrographic analyses define that the dykes and sills consists of basanite, tephrite and phonolitic tephrite. These rocks are composed of olivine, augite, diopside, lamprobolite, nepheline and plagioclase. The mineral characteristic of basanite is olivine that forms reaction rim with pyroxene and lamprobolite. The metasomatic minerals are calcite and siderite invading the matrix and modifying the composition of the rock associated to chlorite and phlogopite, and cancrinite in replacement of nepheline.
The alkaline magmatism is defined by chemical analysis with LREE enrichment by metasomatism, and a decrease in the content of HREE. They derived from spinel and granatiferous mantle. Additional to this, the rocks are classified as intraplate magmatism, in agreement with the extensional event occurred during the Permian to Cretaceous, exposed in the axis of the Eastern Cordillera.